It is known to use polyisocyanates, preferably in form of aqueous emulsions, to bind composites of lignocellulosic materials, as described, for instance, in the European Patent Application 13 112 or in Italian Patent Application 20027 A/88.
It is also known that the preparation of these composites requires a moulding operation on suitable presses between two heated metal plates.
During moulding, the polyisocyanic binder gives, as well known, in the absence of suitable interventions, adhesion phenomena of the manufactured articles to the plates of the press, causing process difficulties.
To overcome this drawback, different solutions have been suggested.
For instance, the use of polyisocyanates together with compounds, such as metal salts of carboxylic acids able to catalyze the formation of isocyanurates from isocyanates, has been described in the U.S. Pat. No. 3,870,665.
Another possibility, illustrated in the most part of the patent literature, consists in using "release agents", that is additives specifically intended to favour the release of the manufactured articles from the metal plates of the press.
In particular: Canadian Patent 1,123,817 suggests the use of phosphates to be added to the polyisocyanic emulsion, whereas Canadian Patent 1,176,778 suggests the use of a polyisocyanic emulsion containing paraffines and salts of higher fatty acids.
German Patent Application 2,921,726 suggests the addition of C.sub.9 -C.sub.18 alkylbenzenesulfonic acids to the polyisocyanic emulsion.
German Patent Application 3,108,537 suggests the use of esters of orthophosphoric acid, and the European Patent Application 46 014 suggests the use of particular waxes having a melting point lower than 70.degree. C., to be dispersed in the aqueous emulsion or to be applied before or after the binding agent.
U.S. Pat. No. 4,257,996 suggests the use of special phosphates or thiophosphates, to be separately added either in the polyisocyanic emulsion or in the polyisocyanate before emulsifying it, and U.S. Pat. No. 4,100,328 suggests the use of a binding agent containing a polyisocyanate mixed with a polyether polyol, having a functionality higher than 2, in the ratio from 0.5:1 to 1:1.
Finally, U.S. Pat. No. 4,490,518 suggests the use of 2-60% of diesters of bicarboxylic acids mixed with the polyisocyanate.
Concerning the modalities for the addition of the release agent, the following addition possibility are reported in the above mentioned patent documents: addition to the polyisocyanate, before or after the emulsifying operation ("inner" addition), or directly on the lignocellulosic material or on the plates of the press ("outside" addition).
It is known that the inner addition of the release agent generally causes a decay of the physico-mechanical properties of the composites obtained, and the outside addition involves working difficulties on the industrial plants.
A solution which overcomes the drawbacks of the prior art has been suggested in the European Patent Application 173,866, wherein the use is described of a binding system containing polyisocyanates not dispersed in water combined with release agents, consisting of the reaction product of a fatty acid containing a high number of C atoms with a compound having at least two reactive hydrogen atoms, such as alcohols and amines.
However, the solution suggested in this patent application is not free of drawbacks in that the advantages relating to the use of the aqueous emulsion are lacking.
These advantages, explained in German Patent Application 2,538,999 and in the European Patent Application 7,532 can be summarized as follows:
better distribution of the binding agent on the chip and therefore better technologic properties of the panel;
possibility to clean the apparatus with water;
possibility of working with chips having a variable moisture content.